In a power system, when a power flow increases suddenly or a considerable imbalance between demand and supply of reactive power occurs, there may be the case where the system voltage drops considerably. When the system voltage drops, an attempt is made to recover the system voltage to stably maintain the system voltage by the reactive power control by a generator linked to the power system or by the reactive power supply by a phase modifying equipment.
On the other hand, an increase in system load also causes the system voltage to drop and there may be the case where, for example, an increase in air-conditioning load at the power peak in summer causes the system voltage to drop considerably. The air-conditioning load has the same characteristic as that of an induction motor load, in which when the terminal voltage drops below a certain value, an action is produced to try to maintain the effective power at a constant level and an input current is increased. Therefore, if the terminal voltage drops due to an increase in air-conditioning load, the reactive power consumed increases suddenly and the system voltage of the power system drops all the more due to a sudden increase in reactive power, and it becomes difficult to stably maintain the system voltage.
As a measure to avoid a system voltage drop due to an increase in the load of a power system, a part of loads of the power system is shed when the system voltage drops below a predetermined value. In this case, depending on how a predetermined value of the system voltage is selected, there may be the case where unnecessary load shedding is performed at the time of regular voltage variations in the system voltage or of large electric power oscillation.
In view of this, as a measure to avoid a voltage drop resulting from the reactive power characteristic of an air-conditioning load, Japanese Patent Application Laid-open No. Hei 4-109818 has disclosed a system in which a load shedding command is issued when both the condition that the rate of change and the amount of change of the admittance of the system load are more than respective predetermined values and the condition that the bus voltage is less than a predetermined value are met continuously for a predetermined period of time. This enables early determination of voltage drop phenomenon and also appropriate detection of a gradual voltage drop phenomenon.
However, in a power system in which a plurality of bus-bars is connected by transmission network, high system loads are connected to the entire power system and power is supplied from each of the bus-bars, therefore, it is practically impossible to find the admittance of the system loads of the entire power system.
In the power system in which a plurality of bus-bars is connected by the transmission network, the respective system loads are connected to the respective bus-bars and power is supplied to the respective system loads from the respective bus-bars. Further, variations in the voltage of the bus-bar occur among the respective bus-bars or power is transmitted and received among the bus-bars. Therefore, even if connected by the transmission network, there are variations in various electrical characteristics among the respective bus-bars and it is difficult to handle various kinds of electric quantity as a single power system.
An object of the present invention is to provide a power system protection system capable of detecting a voltage drop resulting from the reactive power characteristic with high sensitivity and without erroneous determination and of suitably performing an protective operation even in a power system in which bus-bars are connected by transmission network.